Current limiting device

ABSTRACT

A current limiting device as contained in a two-part housing, each part including a terminal. An electrical conductor is mounted between the terminals and has the characteristic of vaporizing with increasing current flow through the conductor. An insulator partially encloses a portion of the conductor and is itself surrounded by the conductor. An insulating member separates each part of the two-part housing and the inner wall of the juncture of the two-part housing is engaged by the periphery of the insulating member which also engages the outer surface of the insulator for retaining the insulator, the conductor surrounding the insulator exerting a compressive stress onto the surfaces of the insulator which opposes the tensile stresses generated therein as a result of pressure developed from vaporization of the conductor.

Brandhorst [451 Sept. 16, 1975 CURRENT LIMITING DEVICE [75] Inventor:Willem Brandhorst, l-lengelo, Netherlands [73] Assignee: HazemeijerB.V., Hengelo,

Netherlands [22] Filed: Aug. 2, 1974 [21] Appl. No.: 494,360

[30] Foreign Application Priority Data 1' 'l'li Primary ExaminerC. L.Albritton Attorney, Agent, or Firm-Watson, Cole, Grindle & Watson [57]ABSTRACT A current limiting device as contained in a two-part housing,each part including a terminal. An electrical conductor is mountedbetween the terminals and has the characteristic of vaporizing withincreasing current flow through the conductor. An insulator partiallyencloses a portion of the conductor and is itself surrounded by theconductor. An insulating member separates each part of the two-parthousing and the inner wall of the juncture of the two-part housing isengaged by the periphery of the insulating member which also engages theouter surface of the insulator for retaining the insulator, theconductor surrounding the insulator exerting a compressive stress ontothe surfaces of the insulator which opposes the tensile stressesgenerated therein as a result of pressure developed from vaporization ofthe conductor.

4 Claims, 3 Drawing Figures v The invention relates toa' currentlimitingadevice, q

comprising "'a' housiiig with two .teminaLmmbers. 3

conductor mounted. within .thehousing. between .the terminal members;The conductor. athhigh current flow tween. The annular member'engagesthe inner edge of the-insulating body. Y I

If the device according to present invention operates in response to; ahigh curren-taflow-,-the ceramic tube is exhibits a resistance increasesuch that the, current flow will be limited, whichconductor is ,at leastp r i lly 9n;

closed in an insulator. Structure is provided between the inner wall ofthe housing and-the outer wall of the insulator, which exerts externalprestressing forces onto the insulator, whereby compressive stresses arecaused in the insulator material, which are oppositely directed to thetensile stresses generated therein due to the pressure developed inthe'portion of the conductor enclosed in the insulator at high currentflow. A device of this kind is known from the German Auslegeschrift No.2,162,991.

The known device is a current limiter, which operates at a predeterminedcurrent flow therethrough, at which the conductor material evaporatesdue to the ohmic losses occurring therein, whereby the electricresistance of the current limiter will increase considerably and thecurrent flow will be limited to such an extent that the current can beswitched off by means of a switching apparatus of relatively smallswitching capacity connected in series with the current limiting device.The conductor is partially enclosed in a tubular insulator of ceramicmaterial, and if the conductor material which is present in theinsulator evaporates, a very high pressure is generated therein, so thathigh tensile stresses will arise in the ceramic material of theinsulator. Since ceramic material is relatively weak for tensilestresses but on the contrary very strong for compressive stresses, theinsulator in the known device is put under a high prestress, so that ifthe device operates still a compressive stress or a very small tensilestress will be present in the ceramic material. The prestress isachieved by applying an insulating filling material between the tubularinsulator and the surrounding housing, the said filling material ispoured in the interspace at relatively high temperature. Then theassembly is cooled down to room temperature. Since the thermal expansioncoefficient of the filling material is larger than that of the ceramicmaterial of the insulator, and the thermal expansion coefficient of thehousing in turn is larger than that of the filling material, acompressive stress is built up in the ceramic insulator and a tensilestress is built up in the housing during the cooling process of theassembly. As a consequence the housing is relatively highly stressed,also under normal circumstances, and the tensile stress in that housingwill increase considerably if the current limiter operates, so that thehousing has to be made very heavy.

The object of the invention is to eliminate such a disadvantage and toprovide a current limiter, of which the housing under normalcircumstances is not stressed and the prestress exerted on the insulatoris built up if the current limiter operates.

The invention provides a device of the kind stated above, wherein theinsulator is arranged practically completely in the material of theconductor and at its exterior is provided with an annular insulatingmember, which engages the inner wall of the housing. The housingconsists of two parts, which are secured to one another with aninsulating body sandwiched therebepractically completelyer'nbedded in aliquid or gaseous conductor material, so that the .ceramic material ofthe insulator is compressively, loaded from all sides. The surroundinghousing need only be capable of resisting that pressureand neednotbedimensioned for a constant'prestressi 1 1 l r 'The device may bearranged so that the insulator is tubular shaped and the'annular mernberis an O-ring, which is arranged a'pproximately'in the middle of theinsulator. As an alternative it is also p ossibleto locate the O-ring atone end of the insulator.

The invention Willnow be explained further in reference to the drawing,in which:

FIG. 1 is a longitudinal section of the device according to presentinvention,

FIG. 2 is a longitudinal section of an other embodiment, and

FIG. 3 is a cross section according to the line IIIIII in FIGS. 1 and 2.

Referring to FIG. 1 the housing of the current limiter consists of twometallic cylindrical parts 1 and 2, which are secured to one anotherthrough an intermediate disc of insulated material 3. The securing meanshave not been illustrated, but they may take the form of draw bolts. Inthe bore of the cylindrical housing is positioned a ceramic tube 4,which is practically completely surrounded by the conductor material 7.The conductor material 7 may be a liquid metal, such as mercury, analkaline metal or any other metal, which in liquid condition exhibits asufficient increase of resis- .tance. The tube 4 is retained in the boreby the O-ring 5, which surrounds the exterior of the tube 4, and alsoengages the inner edge of the disc 3. A part of the bore serves asexpansion space 8, and the conductor material 7 is sealed by means of apiston 6. The space 8 may be filled with nitrogen for example. At theends of the housing parts 1 and 2 the respective terminal members 9 and10 are provided, which in present case take the form of bolts.

If the device is connected into an electric circuit, an electric currentwill flow from the terminal member 9, through the housing part 1, theconductor material 7 in the bore of the tube 4, the housing part 2 andthe terminal member 10. The dimensions of the bore in the tube 4 and thenature of the conductor material 7 determine the magnitude of thecurrent flow, at which the current limiter will operate. As soon as thishappens, a high pressure is generated in the conductor material 7 in thebore, and this high pressure immediately propagates to the areas in theconductor material 7 at the exterior of the tube 4, so that the tube 4is loaded with compressive forces from all sides.

In FIG. 2 another embodiment of the device according to presentinvention is illustrated, and the same reference numerals are used as inFIG. 1. The difference with the first embodiment is that the ceramictube 4 is retained by the Oring, not as in FIG. 1 in the middle but atone of its ends.

What is claimed is:

l. A current limiting device, comprising:

a two-part housing each including a terminal member;

i an insulating member separating each part of said two-part housing,the periphery of saidinsulating member engaging the inner wall of thejuncture of said two-part housing and said insulating member retainingsaid insulator, said conductor surrounding said insulator for exertingcompressive stresses onto the surfaces of said insulator for opposingthe tensile stresses generated within the insulator as a also engagingthe outer surface of said insulator for 1 result of the pressuredeveloped from said conductor at increased current flow; -2. Acurrentlimiting-device as in claim 1 wherein said insulator is tubular shapedand said insulating member includes an O-ring mounted ,in substantiallythe middle region ofsaidinsulat'or. 1 I

31 A current limiting device as in claim 1 wherein said insulator istubular shaped and said insulating member includes anO-ring mounted atthe end portion of said insulator. v a 4. A current limiting device asin claim 1 further comprising a gas-filled chamber at one end of saidconductor and a piston separating said gas-filled chamber and saidconductor.

1. A current limiting device, comprising: a two-part housing eaCh including a terminal member; an electrical conductor mounted between each of said terminal members and having an increasing resistance characteristic with increasing current flow whereby said conductor vaporizes; an insulator for at least partially enclosing a portion of said conductor and surrounded by said conductor; and an insulating member separating each part of said two-part housing, the periphery of said insulating member engaging the inner wall of the juncture of said two-part housing and said insulating member also engaging the outer surface of said insulator for retaining said insulator, said conductor surrounding said insulator for exerting compressive stresses onto the surfaces of said insulator for opposing the tensile stresses generated within the insulator as a result of the pressure developed from said conductor at increased current flow.
 2. A current limiting device as in claim 1 wherein said insulator is tubular shaped and said insulating member includes an O-ring mounted in substantially the middle region of said insulator.
 3. A current limiting device as in claim 1 wherein said insulator is tubular shaped and said insulating member includes an O-ring mounted at the end portion of said insulator.
 4. A current limiting device as in claim 1 further comprising a gas-filled chamber at one end of said conductor and a piston separating said gas-filled chamber and said conductor. 